A 26 year-old man presents to his primary physician with complaints of intermittent crampy epigastric pain, nausea, and vomiting sholy after eating. He usually vomits undigested food and this relives the pain and nausea. He has preferred to eat soft foods his whole life, and he often relies on liquids. He is otherwise healthy and well developed upper gastrintesrinal barium study reveals a markedly narrowed second poion of the duodemun laboratory studies are unremarkable. Probable diagnosis is?